Photovoltaic devices including nitrogen-containing metal contact

ABSTRACT

A photovoltaic cell can include a nitrogen-containing metal layer in contact with a semiconductor layer.

CLAIM FOR PRIORITY

This application claims priority under 35 U.S.C. §119(e) to ProvisionalU.S. Patent Application Ser. No. 60/864,709 filed on Nov. 7, 2006, whichis hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to photovoltaic cells.

BACKGROUND

During the fabrication of photovoltaic devices, layers of semiconductormaterial can be applied to a substrate with one layer serving as awindow layer and a second layer serving as the absorber layer. Thewindow layer can allow the penetration of solar energy to the absorberlayer, where the optical energy is converted into electrical energy.Some photovoltaic devices can use transparent thin films that are alsoconductors of electrical charge. The conductive thin films can be atransparent conductive oxide (TCO), such as fluorine-doped tin oxide,aluminum-doped zinc oxide, or indium tin oxide. The TCO can allow lightto pass through a substrate window to the active light absorbingmaterial and also serves as an ohmic contact to transport photogeneratedcharge carriers away from the light absorbing material. A back electrodecan be formed on the back surface of a semiconductor layer. The backelectrode can include electrically conductive material, such as metallicsilver, nickel, copper, aluminum, titanium, palladium, or any practicalcombination thereof, which is sufficiently strong to provide support forthe semiconductor layer.

SUMMARY

In general, a photovoltaic cell can include a transparent conductivelayer, a first semiconductor layer, a light-absorbing semiconductor, thesubstrate supporting the semiconductor layers, a nitrogen-containingmetal layer in contact with the semiconductor layer and additionalconductive layers for electrical current transport.

A nitrogen-containing metal layer can include an aluminum nitride, amolybdenum nitride, a nickel nitride, a titanium nitride, a tungstennitride, a selenium nitride, a tantalum nitride, or a vanadium nitridein contact with the semiconductor layer. A nitrogen-containing metallayer can include a stoichiometric nitride. In certain embodiments, anitrogen-containing metal layer can include a non-stoichiometricnitride, for example, a nitrogen-deficient nitride.

In one aspect, a photovoltaic cell can further include a plurality ofmetal layers, such as a chromium layer and an aluminum layer, thealuminum layer positioned between the chromium layer and a nitrogencontaining metal layer. In one aspect, the photovoltaic cell can alsoinclude a chromium layer, a molybdenum nitride, and an aluminum layer inbetween the chromium layer and the molybdenum nitride (Cr/Al/MoN).

In another aspect, a system for generating electrical energy can includea multilayered photovoltaic cell, the photovoltaic cell including asubstrate, a transparent conductive layer, a first semiconductor layer,a light-absorbing semiconductor, a nitrogen containing metal layer incontact with the semiconductor layer, additional conductive layersconnected to the nitrogen containing metal layer for electrical currenttransport, a first electrical connection connected to the transparentconductive layer, and a second electrical connection connected to theadditional conductive layers.

In another aspect, method of manufacturing a photovoltaic cell caninclude placing an absorber semiconductor layer on a substrate anddepositing a metal layer on the absorber semiconductor layer onsubstrate in the presence of nitrogen. A method of manufacturing aphotovoltaic cell can further include placing a capping layer betweenthe transparent conductive layer and the first semiconductor layer bydepositing a thin layer on the transparent conductive layer.

A first semiconductor layer can include a binary semiconductor. Thefirst semiconductor layer can include a Group II-VI semiconductor, suchas CdS or CdTe. The photovoltaic cell can include a second semiconductorlayer over the first semiconductor layer. The second semiconductor layercan include a binary semiconductor. The second semiconductor layer caninclude CdTe.

The photovoltaic cell can include a capping layer, the capping layerchemically isolating the transparent conductive layer from thesemiconductor layer. The capping layer can isolate the transparentconductive oxide layer from contact with a semiconductor layer.

The transparent conductive layer can include a transparent conductiveoxide. The transparent conductive oxide can include a tin oxide.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a substrate with multiple layers.

FIG. 2 is a schematic of a substrate with multiple layers.

DETAILED DESCRIPTION

A photovoltaic cell can include a transparent conductive layer on asurface of the substrate, a first semiconductor layer, the substratesupporting the semiconductor layer, and a nitrogen-containing metallayer in contact with the semiconductor layer.

A nitrogen-containing metal layer can include an aluminum nitride, amolybdenum nitride, a nickel nitride, a titanium nitride, a tungstennitride, a selenium nitride, a tantalum nitride, or a vanadium nitridein contact with the semiconductor layer. A nitrogen-containing metallayer can include a stoichiometric nitride. In certain embodiments, anitrogen-containing metal layer can include a non-stoichiometricnitride, for example, a nitrogen-deficient nitride.

In other circumstances, a nitrogen-containing metal layer can include analuminum nitride, a nickel nitride, a titanium nitride, a tungstennitride, a selenium nitride, a tantalum nitride, or a vanadium nitridein contact with the semiconductor layer.

In one embodiment, a photovoltaic cell can further include a pluralityof metal layers, such as a chromium layer and an aluminum layer, thealuminum layer positioned between the chromium layer and a nitrogencontaining metal layer. In another embodiment, the photovoltaic cell canalso include a chromium layer, a molybdenum nitride, and an aluminumlayer in between the chromium layer and the molybdenum nitride(Cr/Al/MoN).

A metal layer can include a single elemental layer or layers frommultiple elements to increase photovoltaic cell efficiency. A metallayer can include rhodium, platinum, palladium, niobium, gold, lead,iridium or tin, or alloy thereof, for example.

Depositing a metal layer that is reacted with nitrogen during sputteringcan increase the resistivity of the metal layer, resulting in anincrease in photovoltaic cell efficiency. Thus, the amount of electricalenergy produced as a percentage of the incident solar energy can beincreased as a result of incorporating nitrogen into a metal layer.Incorporation of nitrogen into a metal film can be achieved by using anitrogen gas supply along with argon in the sputtering system or using apremixed nitrogen/argon gas cylinder. Similar results can be achieved byusing a compound target that is mixed with desired levels of nitrogenand sputtered in argon or argon/nitrogen ambient. The range of nitrogen,for a nitrogen and argon mixed ambient, can be as low as 1-2% to as highas 50% or 100% with no argon. The level of nitrogen used in the mixtureaffects the amount of nitrogen incorporation into the metal film. Onecan thus deposit a metal layer with various levels of nitrogen rangingfrom as low as a few atomic percent to close to stoichiometric or evengreater than 50 atomic percent. Metal deposition can be carried outwithout any intentional heating of the substrate. Substrate heating,however, is one of the primary process variables that is known to affectfilm properties including incorporation of gas-phase impurities such asnitrogen.

A common photovoltaic cell can have multiple layers. The multiple layerscan include a bottom layer that is a transparent conductive layer, acapping layer, a window layer, an absorber layer and a top layer. Eachlayer can be deposited at a different deposition station of amanufacturing line with a separate deposition gas supply and avacuum-sealed deposition chamber at each station as required. Thesubstrate can be transferred from deposition station to depositionstation via a rolling conveyor until all of the desired layers aredeposited. Additional layers can be added using other techniques such assputtering. Electrical conductors can be connected to the top and thebottom layers respectively to collect the electrical energy producedwhen solar energy is incident onto the absorber layer. A top substratelayer can be placed on top of the top layer to form a sandwich andcomplete the photovoltaic cell.

The bottom layer can be a transparent conductive layer, and can be, forexample, a transparent conductive oxide such as tin oxide or tin oxidedoped with fluorine. Deposition of a semiconductor layer at hightemperature directly on the transparent conductive oxide layer canresult in reactions that negatively impact of the performance andstability of the photovoltaic device. Deposition of a capping layer ofmaterial with a high chemical stability (such as silicon dioxide,dialuminum trioxide, titanium dioxide, diboron trioxide and othersimilar entities) can significantly reduce the impact of these reactionson device performance and stability. The thickness of the capping layershould be minimized because of the high resistivity of the materialused. Otherwise a resistive block counter to the desired current flowmay occur. Capping layers are described, for example, in U.S. PatentPublication 20050257824, which is incorporated by reference in itsentirety.

The thickness of the capping layer can be from greater than about 10 Å.In certain circumstances, the thickness of the capping layer can be lessthan about 500 Å. For example, the thickness of the capping layer can begreater than 20 Å, greater than 50 Å, greater than 75 Å or greater than100 Å. For example, the thickness of the capping layer can be less than250 Å, less than 200 Å, less than 150 Å, less than 125 Å, less than 100Å, less than 75 Å or less than 50 Å. Complete coverage of thetransparent conductive oxide layer may not occur. The capping layer canreduce the surface roughness of the transparent conductive oxide layerby filling in irregularities in the surface, which can aid in depositionof the window layer and can allow the window layer to have a thinnercross-section. The reduced surface roughness can help improve theuniformity of the window layer. Other advantages of including thecapping layer in photovoltaic cells can include improving opticalclarity, improving consistency in band gap, providing better fieldstrength at the junction and providing better device efficiency asmeasured by open circuit voltage loss.

The window layer and the absorbing layer can include, for example, abinary semiconductor such as group II-VI, III-V or IV semiconductor,such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO,MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, or mixturesthereof. An example of a window layer and absorbing layer is a layer ofCdS coated by a layer of CdTe. A top layer can cover the semiconductorlayers. The top layer can include a metal such as, for example,aluminum, molybdenum, nickel, titanium, tungsten, or alloys thereof.

Deposition of semiconductor layers in the manufacture of photovoltaicdevices is described, for example, in U.S. Pat. Nos. 5,248,349,5,372,646, 5,470,397, 5,536,333, 5,945,163, 6,037,241, and 6,444,043,each of which is incorporated by reference in its entirety. Thedeposition can involve transport of vapor from a source to a substrate,or sublimation of a solid in a closed system. An apparatus formanufacturing photovoltaic cells can include a conveyor, for example aroll conveyor with rollers. Other types of conveyors are possible. Theconveyor transports substrate into a series of one or more depositionstations for depositing layers of material on the exposed surface of thesubstrate. The deposition chamber can be heated to reach a processingtemperature of not less than about 450° C. and not more than about 700°C., for example the temperature can range from 450-550° C., 550-650° C.,570-600° C., 600-640° C. or any other range greater than 450° C. andless than about 700° C. The deposition chamber includes a depositiondistributor connected to a deposition vapor supply. The distributor canbe connected to multiple vapor supplies for deposition of various layersor the substrate can be moved through multiple and various depositionstations with its own vapor distributor and supply. The distributor canbe in the form of a spray nozzle with varying nozzle geometries tofacilitate uniform distribution of the vapor supply.

The bottom layer can be a transparent conductive layer. A thin cappinglayer can be on top of and at least covering the transparent conductivelayer in part. The next layer deposited is the first semiconductorlayer, which can serve as a window layer and can be thinner based on theuse of a transparent conductive layer and the capping layer. The nextlayer deposited is the second semiconductor layer, which serves as theabsorber layer. Other layers can be deposited or otherwise placed on thesubstrate throughout the manufacturing process as needed.

Referring to FIG. 1, a cross section of the layers of a photovoltaiccell 20 has substrate 210 upon which are deposited the multiple layersused in the photovoltaic cell. The first layer deposited on thesubstrate is a thin film of a transparent conductive layer 220. Thislayer 220 can be a transparent conductive oxide, such as a metallicoxide like tin oxide, which can be doped with, for example, fluorine.Layer 230 can be deposited between the front contact and the firstsemiconductor layer 240, and can have a resistivity sufficiently high toreduce the effects of pinholes in the first semiconductor layer 240.Pinholes in the first semiconductor layer 240 can result in shuntformation between the second semiconductor layer 250 and the firstcontact resulting in a drain on the local field surrounding the pinhole.A small increase in the resistance of this pathway can dramaticallyreduce the area affected by the shunt.

A capping layer 230 can be provided to supply this increase inresistance. The capping layer 230 can be a very thin layer of a materialwith high chemical stability. The capping layer 230 can have highertransparency than a comparable thickness of semiconductor materialhaving the same thickness. Examples of materials that are suitable foruse as a capping layer include silicon dioxide, dialuminum trioxide,titanium dioxide, diboron trioxide and other similar entities. Cappinglayer 230 can also serve to isolate the transparent conductive layer 220electrically and chemically from the first semiconductor layer 240preventing reactions that occur at high temperature that can negativelyimpact performance and stability. The capping layer 230 can also providea conductive surface that can be more suitable for accepting depositionof the first semiconductor layer 240. For example, the capping layer 230can provide a surface with decreased surface roughness. When using atransparent conductive layer 220 and a capping layer 230, the firstsemiconductor layer 240 can be thinner than in the absence of thecapping layer. For example, the first semiconductor layer 240 can have athickness of greater than about 10 nm and less than about 500 nm. Forexample, the first semiconductor layer can have a thickness greater than20 nm, greater than 50 nm, greater than 100 nm, or greater than 200 nmand less than 400 nm, less than 300 nm, less than 250 nm, or less than150 nm.

The first semiconductor layer 240 can serve as a window layer for thesecond semiconductor layer 250. By being thinner, the firstsemiconductor layer 240 allows greater penetration of the shorterwavelengths of the incident light to the second semiconductor layer 250.The first semiconductor layer 240 can be a group II-VI, III-V or IVsemiconductor, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,or mixtures thereof. It can be a binary semiconductor, for example itcan be CdS. The second semiconductor layer 250 can be deposited onto thefirst semiconductor layer 240. The second semiconductor 250 can serve asan absorber layer for the incident light when the first semiconductorlayer 240 is serving as a window layer. Similar to the firstsemiconductor layer 240, the second semiconductor layer 250 can also bea group II-VI, III-V or IV semiconductor, such as, for example, ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs,InSb, TlN, TlP, TlAs, TlSb, or mixtures thereof.

Metal layer 260 can be a nitrogen-containing metal layer. Anitrogen-containing metal layer can include an aluminum nitride, amolybdenum nitride, a nickel nitride, a titanium nitride, a tungstennitride, a selenium nitride, a tantalum nitride, or a vanadium nitridein contact with the semiconductor layer. A nitrogen-containing metallayer can include a stoichiometric nitride. In certain embodiments, anitrogen-containing metal layer can include a non-stoichiometricnitride, for example, a nitrogen-deficient nitride.

In other circumstances, a nitrogen-containing metal layer can include analuminum nitride, a nickel nitride, a titanium nitride, a tungstennitride, a selenium nitride, a tantalum nitride, or a vanadium nitridein contact with the semiconductor layer.

The metal layer 260 can be deposited on a first semiconductor layer 240or on a second semiconductor layer 250.

Referring to FIG. 2, a photovoltaic cell 30 can further include aplurality of metal layers, such as a chromium layer 320 and an aluminumlayer 310, the aluminum layer positioned between the chromium layer anda nitrogen containing metal layer 300. In another aspect, thephotovoltaic cell can also include a chromium layer, a molybdenumnitride, and an aluminum layer in between the chromium layer and themolybdenum nitride (Cr/Al/MoN). A nitrogen containing metal layer 300can be deposited on a second semiconductor layer 550 or on a firstsemiconductor layer 540.

The second semiconductor layer can be deposited onto a firstsemiconductor layer 540. A capping layer 530 can serve to isolate atransparent conductive layer 520 electrically and chemically from thefirst semiconductor layer preventing reactions that occur at hightemperature that can negatively impact performance and stability. Thetransparent conductive layer can be deposited on a substrate 510.

A metal layer can be deposited onto a second semiconductor layer to actas a back metal electrode. A metal layer can be a single elemental layeror layers from multiple elements to increase photovoltaic cellefficiency. The first metal layer that is directly in contact with thetop surface of the second semiconductor can be made more resistive forhigh photovoltaic cell efficiency. The resistivity of the first metallayer can be increased by depositing a metal under nitrogen and argonmixture as an ambient during sputtering. The amount of nitrogenincorporated into the first metal layer, and therefore resistivity, isdependent on the amount of nitrogen available during sputtering. Goodphotovoltaic cell efficiency results can be obtained when the sputteringambient is composed of 13% Nitrogen and 87% Argon. Higher sheetresistance, thus obtained, allows for thinner metal layers, and canthereby reduce manufacturing cost. A first metal layer can include, forexample, chromium, molybdenum, nickel, titanium, selenium or tungsten,or alloys thereof. A first metal layer can have a thickness of less than500 nm, less than 100 nm, or less than 10 nm. A first metal layer canhave a thickness greater than 2 nm and less then 100 nm. Additionallayers can be subsequently deposited on top of the first metal layer toprovide adequate conductivity to transport electrical charges out toexternal load. A second metal layer can include, for example, aluminum,copper, nickel, tungsten, lead, or molybdenum, using argon as asputtering gas. The second metal layer can be as thick as 500 nm to asthin as 100 nm, for example. The second metal layer can be capped with athin layer of less corrosive metal such as chromium to prevent oxidationduring subsequent processing or operation of photovoltaic panels. Thethickness of the capping layer can be as thick as 50 nm to as thin as 10nm.

Devices that contain metal nitrides can improve photovoltaic cellefficiency. Photovoltaic panels containing molybdenum nitrides haveshown an improved efficiency of at least 0.05%, 0.10%, and 0.20%efficiency greater than chromium nitrides. In the following examples,devices were finished with appropriate back contacts. Testing forresults of these devices was performed at initial efficiency, and afteraccelerated stress testing using I/V measurements on a solar simulator.For example, photovoltaic panels containing MoN_(x) had a 0.2%-0.4%point higher efficiency than those containing CrN_(x). Stability ofphotovoltaic panels under accelerated light soak was carried out forcomparison between MoN_(x) and CrN_(x) revealed improved performance ofthe MoN_(x) devices, with no negative impact on metal layer adhesion.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. For example, the semiconductorlayers can include a variety of other materials, as can the materialsused for the buffer layer and the capping layer. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A photovoltaic cell comprising: a substrate; atransparent conductive oxide layer over the substrate; a firstsemiconductor layer over the transparent conductive oxide layer, whereinthe first semiconductor layer includes cadmium; a nitrogen-containingmetal layer in contact with the first semiconductor layer, such that thefirst semiconductor layer is between the nitrogen-containing metal layerand the transparent conductive oxide layer, wherein thenitrogen-containing metal layer comprises one of an aluminum nitride, amolybdenum nitride, a nickel nitride, a tungsten nitride, a seleniumnitride, a tantalum nitride or a vanadium nitride and wherein comprisesgreater than 50 atomic percent nitrogen the nitrogen-containing metallayer; a chromium layer over the nitrogen-containing metal layer; and analuminum layer between the chromium layer and the nitrogen-containingmetal layer.
 2. The photovoltaic cell of claim 1, wherein thenitrogen-containing metal layer includes molybdenum nitride.
 3. Thephotovoltaic cell of claim 1 further comprising a second semiconductorlayer under the first semiconductor layer.
 4. The photovoltaic cell ofclaim 3 further comprising a capping layer, the capping layer chemicallyisolating the transparent conductive oxide layer from the secondsemiconductor layer.
 5. The photovoltaic cell of claim 3 wherein thesecond semiconductor layer comprises a binary semiconductor.
 6. Thephotovoltaic cell of claim 3 wherein the second semiconductor layercomprises CdS.
 7. The photovoltaic cell of claim 3 wherein the firstsemiconductor layer includes a binary semiconductor.
 8. The photovoltaiccell of claim 3 wherein the first semiconductor layer includes CdTe. 9.The photovoltaic cell of claim 4 wherein the capping layer isolates thetransparent conductive oxide layer from contact with the secondsemiconductor layer.
 10. The photovoltaic cell of claim 1 wherein thefirst semiconductor layer includes CdTe.
 11. The photovoltaic cell ofclaim 1 wherein the transparent conductive oxide layer includes tinoxide.
 12. The photovoltaic cell of claim 10 wherein the transparentconductive oxide layer comprises tin oxide.
 13. A system for generatingelectrical energy comprising: a multilayered photovoltaic cell, thephotovoltaic cell including a substrate, a transparent conductive oxidelayer, a first semiconductor layer wherein the first semiconductor layerincludes cadmium, and a nitrogen-containing metal layer in contact withthe first semiconductor layer, such that the first semiconductor layeris between the nitrogen-containing metal layer and the transparentconductive oxide layer, wherein the nitrogen-containing metal layercomprises one of an aluminum nitride, a molybdenum nitride, a nickelnitride, a tungsten nitride, a selenium nitride, a tantalum nitride or avanadium nitride and wherein comprises greater than 50 atomic percentnitrogen the nitrogen-containing metal layer; a chromium layer over thenitrogen-containing metal layer; an aluminum layer between the chromiumlayer and the nitrogen-containing metal layer; a first electricalconnection in electrical communication with the transparent conductiveoxide layer; and a second electrical connection in electricalcommunication with the nitrogen-containing metal layer.
 14. The systemof claim 13 wherein the nitrogen-containing metal layer includes themolybdenum nitride.
 15. The system of claim 13 further comprising asecond semiconductor layer under the first semiconductor layer.
 16. Thesystem of claim 15 wherein the second semiconductor layer comprises abinary semiconductor.
 17. The system of claim 15 wherein the secondsemiconductor layer comprises CdS.
 18. The system of claim 15 whereinthe first semiconductor layer includes a binary semiconductor.
 19. Thesystem of claim 15 wherein the first semiconductor layer includes CdTe.20. The system of claim 15 further comprising a capping layer, thecapping layer isolating the transparent conductive oxide layer fromcontact with the second semiconductor layer.
 21. The system of claim 13wherein the first semiconductor layer includes CdTe.
 22. The system ofclaim 13 wherein the transparent conductive oxide layer includes tinoxide.
 23. A photovoltaic cell comprising: a substrate; a transparentconductive layer over the substrate; a first semiconductor layer overthe transparent conductive layer, wherein the first semiconductor layerincludes cadmium; a second semiconductor layer over the firstsemiconductor layer, wherein the second semiconductor layer includesCdTe; a molybdenum nitride layer in contact with the secondsemiconductor layer, such that the second semiconductor layer is betweenthe molybdenum nitride layer and the first semiconductor layer andwherein comprises greater than 50 atomic percent nitrogen molybdenumnitride layer; a chromium layer over the molybdenum nitride layer; andan aluminum layer between the chromium layer and the molybdenum nitridelayer.